The incident took place at around 8:30 p.m. at the Aviary, located at 955 W. Fulton Market. Sources said Eric Trump was a patron at the cocktail lounge when an employee approached and spit on him.

Chicago police responded to the scene, with CPD spokesman Anthony Guglielmi tweeting just before 10:30 p.m. that officers were "assisting the United States Secret Service with a law enforcement matter," adding that any further inquiries "regarding a federal protectee" should be directed to the Secret Service. The Secret Service declined to comment.

Eric Trump was seen leaving the bar in a motorcade of two black SUVs. In a statement to NBC News, he called the incident "purely a disgusting act by somebody who clearly has emotional problems.”

Trump had earlier tweeted photos of the Trump Hotel in Chicago at around 10:45 p.m. Tuesday, with the caption "Always love visiting @TrumpChicago⁩! It is one of the most beautiful buildings in the world! @TrumpHotels," but made no mention of the alleged incident.

The Aviary said in a statement Wednesday that the employee involved in what it called an "unfortunate incident" was put on leave.

"We did not witness the incident and we are just beginning to learn the details," the bar said. "What is certain is this: no customer should ever be spit upon."

The Aviary said it has received threatening comments and fake reviews online in the aftermath of what happened.

It was not immediately clear if anyone was taken into custody in connection with the incident, or if charges would be filed.